Chemical and biological separations are routinely performed in various industrial and academic settings. One technique for performing such separations, chromatography, encompasses a number of methods that are used for separating closely related components of mixtures. In fact, chromatography has many applications including separation, identification, purification, and quantification of compounds within various mixtures. Chromatography is a physical method of separation wherein components typically partition between two phases: a stationary phase and a mobile phase. Sample components are carried by a mobile phase through a bed of stationary phase.
In column chromatography, the stationary phase refers to a coating on a solid support that is typically contained within a tube or other boundary. The mobile phase is forced by gravity or a pressure differential through the stationary phase. The mobile phase acts as a carrier for a sample solution. As the sample solution flows with the mobile phase through the stationary phase, the components of that solution will migrate according to interactions with the stationary phase and are retarded to varying degrees. The time a particular compound spends in the stationary phase relative to the fraction of time spent in the mobile phase will determine its velocity through the column.
Separation columns may be packed in several different ways, although conventional methods for packing such columns are typically slow and difficult. A simple packing method is to dry-pack an empty tube by shaking particles down with the aid of vibration from a sonicator bath or an engraving tool. A cut-back pipette tip may be used as a reservoir at the top, and the tube to be packed is plugged with parafilm or a tube cap at the bottom. The dry-packed tube may then be secured at the bottom end with a ferrule, frit, and male nut, and at the top end with the same fittings, minus the frit. The tube contents may be further compressed by flowing pressurized solvent through the packing material. When compacting of the particle bed has ceased and the fluid pressure has stabilized, the tubing is cut down to the bed surface, and then reassembled before use.
Another packing method utilizes slurry. An empty column is attached to a packing reservoir such as a Poros® Self-Pack® reservoir (PerSeptive Biosystems, Foster City, Calif.) upon which the column is filled with an appropriate amount of dilute slurry. The end of the reservoir column is then screwed on firmly before the tube is internally pressurized with a fluid and an appropriate instrument such as a pump. Pressures of several hundreds or even thousands of pounds per square inch (psi) may be applied, depending on the material properties of the tubing and the ability to seal the apparatus from leakage. Typically, a packed tube is cut following the packing step to remove any dead volume (where packing is incomplete or not present), to remove any contaminated regions, and/or to yield multiple sections of desired length. Thereafter, fittings are added to each tube sections to permit interface with other fluidic components such as pumps.
The foregoing packing methods have drawbacks that limit their utility. To begin with, such methods are relatively slow and inefficient. Conventional dry packing and slurry packing methods typically require tubing to be cut or trimmed, and then fitted with fittings for connecting to other components. These steps are labor-intensive, and the presence of additional fittings presents potential leakage problems during operation. Additionally, conventional slurry-packing methods are plagued with notorious blockage problems, especially when applied to small-bore columns such as capillaries. Such blockage or clogging during the packing step can prevent a column from being packed completely, if at all.
Also, it may be desirable to include multiple separation columns in a single device, such as a microfluidic device. Such an arrangement would allow high throughput analysis of samples by analyzing multiple samples in parallel. Conventional packing methods, however, are not capable of packing multiple separation columns simultaneously. Moreover, it may be desirable to pack several such microfluidic devices simultaneously to permit the fabrication of large numbers of such devices.
In light of the foregoing, there exists a need for improved column packing methods. It would be desirable to provide multiple separation columns on a single device, such as a multi-column microfluidic separation device, and to provide methods for fabricating such devices. It also would be desirable to provide packing methods that may be easily scaled up to permit fabrication of separation devices in large quantities.